I finally managed to get this working on Windows XP
Probably I will write later a full guide

meanwhile, here is how I did it:
*I'm using widcomm bluetooth v 5.0, and I have Network access enabled with "allow other devices to access the internet/lan via this computer" selected

*I paired the computer with the n800

*Instead of Internet Connection Sharing in windows, I made a bridge between my lan connection and the bluetooth connection, I'm not even sure at this point if its necessary.

*on N800, I installed rootsh(to gain root access) and used the script from http://www.internettablettalk.com/fo...8&postcount=42
make sure you change the mac adress and name
(windows users beware of file encoding, the script should be in unix format (i have no idea), I used notepad++ to change it's encoding)

*you neead maemo-pam 1.0.1 wich unfortunately is unavailable in maemo garage, just add this repository http://www.gronmayer.com/it/dl.php?id=243 and you should be able to install maemo-pam 1.0.1

*now, just connect to the dummy connection, and start the script

I'm still looking for an easy and automated way to get this connection done